1. Field of the Invention
The invention relates to a global positioning system (GPS) receiver with extended hot start capability. In particular, the GPS receiver is capable of being restarted in 1-second hot-start mode with an extended power-off period.
2. Description of Related Art
A Global position system (GPS) consists of 28 satellites that circumnavigate along six orbital planes. Each GPS satellite transmits a pseudorandom code modulated signal with a chip rate of 1.023 MHz at a carrier frequency of 1574.42 MHz.
A GPS receiver comprises a high precision clock circuit driven by a high precision clock reference, which also drives GPS correlator circuitry comprising of multiple correlators. When transmission time of satellite signals are measured from more than four correlator channels, each correlator channel tracking different satellites signals, position of the GPS receiver and time offset of the precision clock can be derived. Once the time offset is derived, the high precision clock circuit is synchronized to match an atomic clock synchronized GPS clock, having accuracy within several tens of a nanosecond.
When powering up a GPS receiver, it goes through the process of signal acquisition, signal tracking, data bit synchronization and frame synchronization to extract desired GPS data for position fix. The GPS data is organized into 5 sub-frames, with each sub-frame spanning 6 seconds. Ephemeris data for calculating satellite position is within sub-frames 1-3. Every four hours, new ephemeris data is uploaded to the satellites and these new data need to be downloaded to calculate the satellite position.
When turning on a GPS receiver that has been powered off for several hours, a waiting time is required to download the ephemeris data and it takes about 18 to 30 seconds; the GPS receiver cannot generate the position during this data decoding time.
GPS receivers typically are implemented capable of supporting hot-start, this applies when a GPS receiver has been restarted and has not been powered off for more than an hour. Battery-sustained, internally held ephemeris data is still valid and the position can be calculated without needing to download new ephemeris data, thereby eliminating the waiting time of 18 to 30 seconds. Only couple seconds of acquiring signal, tracking signal, perform data and frame synchronization is needed to generate position fix.
With reference to FIG. 2, the partial circuit of a conventional GPS receiver comprises a radio frequency (RF) chip (70), a baseband chip (50) and a rechargeable backup battery (60). After the GPS signal is received and processed with a series of RF front-end procedures such as signal amplification, noise filtering, and frequency translation and sampling, the processed signal is output to the baseband chip (50) for further processing to generate position velocity and time information.
The baseband chip (50) comprises a real time clock unit (51), a timing control unit (52), a correlator unit (53), a processor unit (54), a memory (55) and a general-purpose input/output (GPIO) unit (56). The real time clock unit (51) is integrated with a backup static random access memory (SRAM) and uses a crystal oscillator to generate a real time clock signal.
The RF chip (70) and the baseband chip (50) each receive an individual operating voltage (VCCA) (VCC1) from a main supply voltage. The main supply voltage is further used to charge the backup battery (60). However, the real time clock unit (51) of the baseband chip (50) is powered by another operating voltage (VCC2), supplied by the backup battery (60) when the main supply voltage is removed, and does not share the operating voltage (VCC1) as other elements of the baseband chip (50).
Therefore, after the GPS receiver is powered off, the real time clock unit (51) can continue working and the ephemeris data stored in the backup SRAM are retained for use when the GPS receiver is restarted while the battery lasts. The 1-second hot start mode requires that the drift of the real time clock be within 0.5 milliseconds, and time accurate to millisecond can be correctly estimated without needing to go through data bit and frame synchronization to extract precise time information. As a result, the GPS receiver can immediately achieve position fix upon locking onto at least 4 GPS satellite signals.
The precision of the crystal oscillator for the real time clock (51) is typically in a large range of +/−25 ppm. In a worst case scenario, the real time clock will drift off by 0.5 millisecond within a very short power-off time, as little as about 20 seconds. Therefore the 1-second hot start mode can only be reliably performed within 20 seconds of GPS receiver powering off time.
To overcome this shortcoming, the present invention provides a GPS receiver with extended 1-second hot start capability.